In a typical coal-fired power plant, coal is burned in a boiler to make high temperature and pressure steam to drive a steam turbine and electricity generator. For the steam-side process, the initial superheated steam drives the high-pressure turbine first for power generation, before returning to the boiler for reheating to bring steam temperature back to over 540° C. (according to the steam cycle specifications and boiler design). The reheated steam then enters intermediate-pressure and low-pressure turbines to generate additional electricity. The steam pipe connecting the intermediate-pressure turbine exhaust and the low-pressure turbine inlet is called the cross-over section, where the steam for solvent regeneration in the CO2 capture process is extracted. The exiting saturated steam from the low-pressure turbine is condensed in a water-to-steam heat exchanger (or condenser). The condensate is pressurized via feedwater pumps, heated via feedwater heaters and economizers, and fed back to the boiler to complete the steam cycle.
For the gas-side process, typically after combustion of the coal in low NOx burners the combustion gases leave the boiler and are treated by a NOx removal technology called Selective Catalytic Reduction (SCR). After the SCR treatment, the gas is further treated in a fly ash removal device, such as an electrostatic precipitator, to remove particulates. After this treatment, the gas is routed through an SO2 removal device (SO2 scrubber or polisher). At this point, the carbon capture process begins.
It is well-known in the art to use CO2 absorbers/scrubbers including scrubbing solvents for capturing CO2 from post-combustion gases, such as from utility flue gases. As an example, a conventional solvent often used is 30 weight percent monoethanolamine (MEA). Likewise, it is known to use solvents comprising piperazine, K2CO3/KHCO3, NH3, and others. In commercial use, the solvents must be regenerated, i.e. have absorbed CO2 removed, for continued use in the systems. All of these processes incur costs in the form of at least energy costs and capital costs for equipment to accomplish them. An industry goal is to provide processes and systems for carbon capture and recovery, such as in commercial scale power plants, with as minimal an energy and capital cost as possible, to provide power at as low a cost as possible.
To address the aforementioned and other issues, and to meet the U.S. Department of Energy's goals of 90% CO2 capture with 95% CO2 purity at a cost of no more than $40/tonne of CO2 captured, a hybrid process is described to achieve high percent CO2 capture from low driving force coal post-combustion flue gas, CO2 purity for compression, and low process energy requirement. The described process incorporates membrane-based CO2 enrichment of post-combustion gases, coupled with a heat-integrated aqueous CO2 capture system and a two-stage solvent regeneration process to provide a desired CO2 purity from utility flue gases.